The present invention relates to a method for determining the position of the rotor of an electric machine in the radial direction and in the axial direction, which method applies a cogged ring, which is placed around the shaft of the rotor. The invention also relates to a position sensor for determining the position of the rotor of an electric machine in radial direction and in axial direction, which position sensor comprises a cogged ring, through which the shaft of the rotor is arranged to be placed.
In magnetic bearings, the position of the rotor is normally measured in the axial direction and, at both ends of the rotor, in two radial directions, that is, with a total of five coordinates. On the basis of this measurement data, the flows of the magnets are controlled in such a way that the rotor is suspended in a desired position. A position sensor is an important part of the magnetic bearing system. The position sensor must be sufficiently accurate and substantially linear. It must have a good signal/noise ratio and a high intrinsic sensitivity. It should be reliable, inexpensive to manufacture, and as easy to install as possible. Furthermore, it is important that each coordinate is measured by at least two sensor parts from different directions to obtain a differential measurement. This will efficiently eliminate distortions caused by thermal expansions and contamination.
German patent application DE 2848173 presents a sensor arrangement for measuring the deviation of the shaft in the axial direction as well as in the radial direction. As the most substantial feature, the arrangement comprises a cogged ring arranged in connection with the shaft. Outside the ring, there is a fixed additional ring, in relation to which the measurement is taken. The width of the cogs in this cogged ring is greater than the width of the auxiliary ring in this axial direction of the rotor. In the cogged ring connected to the rotor, all the cogs are not in the same position in relation to the axial direction of the rotor, but the cogs can be divided into two sets, in which the cogs of the first set are placed mutually substantially in the same position in relation to the axial direction, and the cogs of the second set are placed mutually substantially in the same position and at least partly in a different position than the cogs of the first set, seen in the axial direction. The cogs of the ring are equipped with windings for producing and measuring a magnetic flux. The windings formed in the cogs of the first and second sets of cogs are intended to measure deviations in the radial and axial directions, respectively. When the rotor is in the desired position, the location of the windings measuring in the radial direction in relation to the auxiliary ring is such that the auxiliary ring is at the cogs of the first set. Measurement in the axial direction is arranged in such a way that the cogs of the second set are only partly at the auxiliary ring. Thus, a shift in the axial direction can be detected as a change in reluctance. This solution presented in the German application publication DE 2848173 has, however, some disadvantages in view of its use in connection with magnetic bearings. First of all, the sensor does not measure the axial direction differentially, but only as a change in one reluctance. Thus, the thermal expansion between the rotor and the stator is manifested in the axial measurement result. Thus, to produce an appropriate axial measurement, two such sensors, mounted in parallel, are needed. For measuring all the necessary degrees of freedom, a similar pair of sensors must also be installed at the other end of the rotor. If the differential measurement is implemented by installing only one sensor at each end of the rotor, this will result in a high precision requirement for installing the sensors. Furthermore, such a solution is still sensitive to thermal expansion. Secondly, in the presented solution, the cogs and coils are in the rotor. The cogged rotor will cause air friction at a high speed and has a poor mechanical strength. Furthermore, in the rotor rotating at a high speed, it is difficult to arrange the supply of electricity to the coils on the rotor side.
Another known sensor arrangement for measuring deviations in the radial and axial directions is that the rotor shaft is provided with a conical section. A position sensor is arranged in connection with this conical section. Thus, a deviation in both the axial and the radial directions can be detected in the rotor. This sensor arrangement involves, for example, the disadvantage that the machining of such a cone in the rotor requires extra work stages, and the machining with a sufficient precision is difficult. Between measurements in the radial and axial directions by a position sensor placed in connection with such a cone, there is a so-called cross connection; that is, a displacement of the rotor in, for example, the axial direction is also manifested in the measurement in the radial direction, and vice versa. Thus, to eliminate cross connections, the measurement results must be processed numerically in a relatively complicated way. Also this solution involves a requirement of high precision in the mounting and is sensitive to thermal expansions.
It is an aim of the present invention to provide a method for determining the position of the rotor of an electric machine in the radial and axial directions by using one sensor which is formed in such a way that some of the cogs of the sensor are used for measuring in the radial direction and some are used for measuring in the axial direction, to obtain a differential measurement. Another aim of the present invention is to provide a position sensor in which the above-mentioned drawbacks are eliminated to a large extent. The method according to the present invention is characterized in that an auxiliary ring is provided substantially at the location of a cogged ring on the shaft, that the cogged ring is provided with at least three sets of cogs, wherein the cogs of the first set of cogs are used to measure the movement of the shaft in the radial direction, the cogs of the second and third sets of cogs are used to measure the movement of the shaft in the axial direction, and that the cogs of the second and third sets of cogs are placed, in the axial direction, at least partly in a different location in relation to each other. The position sensor according to the present invention is characterized in that the shaft comprises an auxiliary ring placed substantially at the location of a cogged ring, that the cogged ring comprises at least three sets of cogs, wherein the cogs of the first set of cogs are arranged to be used to measure the movement of the shaft in the radial direction, the cogs of the second and third sets of cogs are arranged to be used to measure the movement of the shaft in the axial direction, and that the cogs of the second and third sets of cogs are placed, in the axial direction, at least partly in a different location in relation to each other. The invention is based on the idea that the position sensor is formed of a cogged ring which is placed around the rotor and the rotor is provided with a section in which the electric and/or magnetic properties differ from the rest of the structure of the rotor shaft, and that the cogs for measuring in the axial direction are arranged, in the axial direction, in at least two locations to make differential measurement possible.
The present invention shows remarkable advantages compared to solutions of prior art. When using the position sensor of the invention, a single sensor can be used to achieve a reliable measurement in the radial and axial directions, in which the effects of temperature can be taken into account better than in position sensors of prior art. Two position sensors according to the invention can be used to produce two separate items of differential information in the axial direction, i.e. one at each end of the rotor. Thus, the thermal expansion between the rotor and the stator can be detected. Furthermore, the position sensor according to the invention can be easily produced by using a sheet cogged in a specific way, combined as a set of sheets. Moreover, to use the position sensor according to the invention, no specific conical forms need to be shaped in the rotor shaft, but a straight shaft can be used.